1. Technical Field of the Invention
The present invention relates generally to spark plugs for internal combustion engines. More particularly, the invention relates to a spark plug which has an improved configuration of ground electrode for ensuring high capability to ignite the air-fuel mixture (referred to simply as ignition capability hereinafter).
2. Description of the Related Art
A conventional spark plug generally includes a tubular metal shell, an insulator, a center electrode, and a ground electrode.
The insulator is retained in the metal shell such that an end of the insulator protrudes from an end of the metal shell. The center electrode is secured in the insulator with an end thereof protruding from the end of the insulator. The ground electrode is fixed to the end of the metal shell and faces the end of the center electrode in the axial direction of the center electrode through a spark gap formed therebetween.
More specifically, the ground electrode is made by bending a rectangular bar to have a substantially “L” shape. The bar has a pair of thickness surfaces that define the thickness of the bar (i.e., the thickness of the ground electrode) and a pair of width surfaces that define the width of the bar (i.e., the width of the ground electrode); the width is greater than the thickness. To facilitate the bending process, the bar is bent to fold the width surfaces at substantially right angle. Consequently, after assembly of the spark plug, each of the width surfaces of the ground electrode has one portion perpendicular to the radial direction of the center electrode and the other portion perpendicular to the axial direction of the center electrode.
When the spark plug is installed in an engine cylinder with the center and ground electrodes aligned in the flow direction of the air-fuel mixture, either of the width surfaces will confront the flow of the air-fuel mixture, thus deteriorating the ignition capability of the spark plug. More specifically, when the ground electrode is located on the upstream side of the center electrode with respect to the flow of the air-fuel mixture, the outer width surface of the ground electrode (i.e., the outer one of the width surfaces with respect to the center electrode) will hamper the flow of the air-fuel mixture, thus making it difficult for the flame to propagate. On the contrary, when the ground electrode is located on the downstream side of the center electrode, the air-fuel mixture will flow along the inner width surface of the ground electrode (i.e., the inner one of the width surfaces with respect to the center electrode) into the air pocket formed between the inner surface of the metal shell and the outer surface of the insulator, thus causing the flame to be extinguished.
To solve the above problem, a variety of ground electrode configurations have been proposed. For example, Japanese Patent First Publication H9-148045 discloses a ground electrode having slits formed through the width surfaces thereof and a ground electrode having diverging portions. However, either of the disclosed ground electrodes has low strength and low heat resistance, and thus can be easily melted down or detached from the metal shell.